In a pneumatic radial tire for heavy load for use in a vehicle such as a construction vehicle adapted to traveling on unpaved roads, there has been widely used a tread pattern having lug grooves in side regions of a tread surface thereof.
Conventionally, a tire of this type is generally designed such that, in a cross section of a lug groove in the width direction thereof, an angle formed by a groove wall on the step-in side of a land portion, with respect to a line normal to a surface of the land portion, is substantially equal to an angle formed by a groove wall on the kick-out side of the land portion, with respect to a line normal to the surface of the land portion. With this design, upon rotation of a tire, when the leading edge 31 of one land portion is in contact with the ground and the trailing edge 32 of another land portion provided adjacent to the one land portion with a lug groove 33 therebetween is still in contact with the ground, as shown in FIG. 5, slippage occurs between the trailing edge 32 of another land portion and the ground as the trailing edge 32 is released from a road surface, whereby uneven wear is quite often generated such that the tailing edge 32 of the another land portion is worn more quickly than the leading edge 31 of the one land portion.
Specifically, the aforementioned slippage is caused by the facts that: when a leading edge 31 of one land portion is brought into contact with a road surface, tread rubber of the leading edge 31 receives a pushing-up force acting on the inner side in the radial direction, whereby a belt section located on the inner side in the radial direction of the leading edge 31 is pressed and deformed on the inner side in the radial direction; due to this deformation of the belt section located on the inner side in the radial direction of the leading edge 31, a belt section of a trailing edge 32 of another land portion separated by a lug groove 33 is deformed toward the outer side in the radial direction by a pressing counterforce O, and this counterforce O elastically deforms a groove wall 35 on the trailing edge side of the another land portion in a direction that narrows a width of the groove; accordingly, as the trailing edge 32 of the land portion leaves the road surface, the trailing edge 32, which has been biased to return to the original shape, makes relatively large slip with respect to the road surface.
In view of the facts described above, there has been proposed a pneumatic radial tire for heavy load, designed such that: in a lateral cross section of a lug groove, a groove wall on a step-in side of one land portion is linearly tilted by a larger angle with respect to a line normal to a surface of the one land portion than a groove wall on a kick-out side of another adjacent land portion is; due to this, some of a pushing-up force acting on the inner side in a radial direction of the one land portion when the leading edge of the one land portion is brought into contact with a road surface is converted into a deformation force acting in a direction widening a width of a lug groove between the land portions; and, as a result, a pressing force acting on the belt section is reduced accordingly, whereby occurrence of partial wear of the trailing edge of the another land portion at an early stage is prevented.
However, in this tire, in a case where the groove width at the bottom of the lug groove is left unchanged, the groove width at a tread surface, of the lug groove, relatively increases and thus the amount of rubber of the land portion relatively decreases, possibly deteriorating wear resistance at the tread surface of the land portion. On the other hand, in a case where the groove width of the lug groove at the tread surface is left unchanged and rather a width of the groove bottom of the lug groove is made small, strains are concentrated at the groove bottom when the tire is rotated, possibly leading to generation of cracks at the groove bottom.